This invention relates to a method and ultrasound apparatus for measuring the velocity of blood flow and especially to bidirectional velocity measurement in a system having quadrature echo signal demodulation.
Blood velocity can be detected by measuring the Doppler shifts in frequency imparted to ultrasound by reflection from moving red blood cells. Physicians employing Doppler methods for diagnostic purposes are interested not only in estimating velocity but also in determining whether blood flow is taking place towards or away from the ultrasonic transducer. This need for bidirectional velocity measurement of blood flow has become well accepted, particularly since it was possible to prove that in certain cases, a forward as well as reverse blood flow may simultaneously take place across a given section of the human circulatory system.
Directional velocity capability in prior art systems has been provided by synchronously detecting the echoes in two quadrature channels to obtain both real and complex vectors of the Doppler shifted frequencies. Echo signals from the two channels have been processed in a number of different ways to allow directional flow waveforms to be displayed. Signal analysis in the time domain has been used as a direction resolving technique; using this technique flow direction is determined from which of the processed echo signals leads in time, and this particular signal is then switched on for analysis and display. The method is susceptible to switching artifacts.
Signal analysis in the phase domain has also been proposed as a method of separating from each other forward and reverse flow components. This method relies on phase shifting by 90.degree. signals in each of the quadrature channels and appropriately summing cross terms to implement the mathematical separation of forward and receding flow components. In practice, deviations from the ideal 90.degree. phase shifts required often produces cross talk betwen forward and reverse channels.
Most Doppler units available at present, however, do not employ quadrature channels and utilize a zero-crossing technique to convert the flow velocity and its direction to a proportional positive or negative analog voltage suitable for display. This technique produces a voltage proportional to the mean velocity in the same volume of blood insonified by the ultrasound beam. For some applications, this is all that is necessary, but often it is desirable to visualize the spectral distribution of velocities present in the sample volume.